Geotectonic significance of gneissic amphibolites from the Vema Fracture Zone, equatorial Mid‐Atlantic Ridge

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Abstract
A large collection of gneissic amphibolites was recovered by two close dredge hauls from the deepest part of the north facing slope of the transverse ridge forming the south wall of the Vema Fracture Zone. Serpentinites and various types of gabbroic rocks ranging from undeformed slightly uralitized gabbros and norites to flaser and mylonitic gabbros were associated with the gneissic amphibolites (in the shallowest of the dredge hauls). Petrological studies indicate that the amphibolites were derived from similar gabbroic rocks that reequilibrated under stress in the conditions of the amphibolite facies. On the other hand, the associated meta‐gabbros display sequences of secondary minerals, indicating complex cooling and cataclastic histories without reaching metamorphic equilibrium. We tentatively suggest that the gneissic amphibolites and associated metagabbros formed in a vertical shear zone generated in oceanic layer 3 by tectonism associated with the Vema Fracture Zone. Hydrothermal circulation of seawater along the highly permeable shear zone was activated by magmatic intrusions. K/Ar dating suggests, within relatively large analytical uncertainties, that the amphibolite metamorphism took place 10 m.y. ago, i.e., at a time when the dredging sites were located in the vicinity of the spreading center.